The kernel of the maize plant, Zea mays, consists of four separate components: the pericarp, or hull, which is the outer covering of the kernel; the germ, or plant embryo; the endosperm, which makes up over 80% of the kernel and which provides energy and protein for the developing embryo; and, the tip cap, which is the attachment point of the kernel to the corncob. Of these components, the endosperm is particularly important because the starch and protein reserves it contains are a major source of food, feed, and industrial raw material, a fact which is true not just for corn, but for cereal plants in general.
Cereal endosperm is generally divided into four major regions with clearly distinct functions: the starchy endosperm, which takes up most of the volume, is a storage organ accumulating reserve substances; the aleurone layer, which is the outer cell layer surrounding the endosperm, stays alive during seed dormancy and is essential for the mobilization of the reserve substances during germination; the basal endosperm transfer layer, which is a specialized part of the outer cell layer in close contact with the vascular bundles of the mother plant, is responsible for high throughput nutrient transfer from source organs into the developing endosperm; and, the embryo surrounding region (“ESR”), which is characterized by small cells rich in cytoplasm, which possibly extends the functions of the suspensor in embryo nutrition. See Bonello et al. (2000) Gene 246:219-227, the contents of which are herein incorporated by reference.
Given the agronomic importance of the endosperm, there is a continued need for techniques for improving the composition or development of this tissue or one of its component regions. One such technique involves the use of expression constructs that comprise a promoter that drives expression of operably linked endogenous or exogenous genes of interest preferentially in the endosperm or one of its component regions. With regard to the ESR region, for example, the ESR-preferred expression of an endogenous or exogenous protein controlling cell division (e.g., a cytokinin biosynthesis gene such as isopentenyl transferase) could be used to increase the number of cells in the ESR, possibly resulting in larger seeds or creating a sink for photosynthate to buffer seed development during transient abiotic stress. Preferential expression of a protein of interest in a particular region of the endosperm may also be useful for affecting surrounding regions. For example, preferential expression in the ESR of genes that are toxic or deleterious to the adjacent embryo (e.g., DAM methylase from E. coli) could be used to damage or destroy the embryo. Additionally, the ESR-preferred expression of an antisense RNA sequence may be used to inhibit the expression of endogenous genes, thereby modulating the development or altering the composition of the endosperm.
In light of the above discussion, it is clear that there is a significant need for plant regulatory elements that preferentially direct gene expression to the endosperm or one of its component regions. The present invention satisfies this need by providing a promoter with activity that is specific to the ESR region of endosperm, that is, an ESR-preferred promoter.